1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device and, more specifically, to a structure of a source diffusion layer region of a non-volatile semiconductor memory device and to a manufacturing method thereof.
2. Description of the Background Art
As a method of isolating each cell region of a non-volatile semiconductor memory device represented by a conventional flash memory, element isolating structure utilizing formation of an isolation film through LOCOS (local oxidation of silicon) process has been dominant. The LOCOS isolation, however, has a limit in miniaturization of the semiconductor devices, and hence STI (Shallow Trench Isolation) comes to be widely used recently. In STI, a trench is formed at a main surface of a semiconductor substrate and filled with a burying oxide film, so as to establish element isolation.
A non-volatile semiconductor memory device having a common trench isolation suffers from a problem that a source diffusion layer region comes to have high resistance when self align source etching is performed utilizing anisotropic dry etching. In a non-volatile semiconductor memory device using LOCOS isolation, an end portion of an isolation film has moderate inclination. Therefore, a source diffusion layer interconnection of low resistance is formed between respective sources at the time of ion implantation, and therefore, such a problem does not arise. In the trench isolation described above, a sidewall of the trench formed at the main surface of the semiconductor substrate has a steep inclination of approximately 90xc2x0. Therefore, it has been difficult to form a source diffusion layer interconnection of low resistance on the sidewall during ion implantation.
In the following, a structure of a non-volatile semiconductor memory device employing the conventional trench isolation as well as the problem mentioned above will be described in greater detail. FIG. 6 is a top view representing a structure of a conventional non-volatile semiconductor memory device, and FIGS. 7 to 10 are cross sections showing various portions of the non-volatile semiconductor memory device.
In the non-volatile semiconductor memory device, when the main surface of the semiconductor is viewed from above, cells forming a memory portion are arranged in a matrix. Along a bit line (BL) direction of the cells arranged in a matrix, a plurality of trench isolation regions 17 are formed as stripes parallel to each other at the main surface of a semiconductor substrate 1. Along a word line (WL) direction, gate regions 15 are formed at the main surface of the semiconductor substrate 1. The gate region 15 includes a floating gate 5 and a control gate 6. A source diffusion layer region 2 and a drain region 4 are formed sandwiching the gate region 15. Drain region 4 is electrically isolated by the above described trench isolation region 17. A drain electrode 12 for taking out electrical charges is formed in each drain region 4. Source diffusion layer region 2 is constituted by individual source regions 2a arranged along the word line direction electrically connected with each other by source diffusion layer interconnections 2b. Source diffusion layer interconnection 2b is formed by removing an isolation film positioned between individual source regions 2a by etching so as to expose a trench surface of semiconductor substrate 1, and by performing ion implantation to the thus exposed trench surface of semiconductor substrate 1. Thus, the source regions 2a aligned in the word line direction are electrically coupled, so that all have the same potential. The aforementioned floating gate 5 is arranged independently for each cell, between source region 2 and drain region 4.
Cross sectional structures of respective regions will be described in detail in the following. First, FIG. 7 is a cross section taken along the line VIIxe2x80x94VII of FIG. 6. Referring to the figure, the main surface of the semiconductor in the source diffusion layer region 2 has recesses and protrusions. This shape results from the above described process in which a trench isolation film 17 provided at the main surface of semiconductor substrate 1 is removed to expose the trench portion. Source diffusion layer region 2 continuously extends immediately below the surface of the recesses and protrusions. Further, an interlayer insulating film 11 is formed to cover the main surface of semiconductor substrate 1.
FIG. 8 is a cross section taken along the line VIIIxe2x80x94VIII of FIG. 6. Referring to this figure, drain regions 4 of the cells are isolated by trench isolation 17 from each other, and on each drain region 4, a drain electrode 12 is formed. Different from the above described source region 2a, drain regions 4 aligned in the word line direction are electrically independent from each other, and therefore, electric charges are taken out from each drain region 4 through drain electrode 12.
FIG. 9 is a cross section taken along the line IXxe2x80x94IX of FIG. 6. Referring to the figure, in the cross section of semiconductor substrate 1 along the direction of extension of a gate region 15, a channel 8 is formed at the main surface of the semiconductor substrate 1 sandwiched between trench isolation regions 17, and a floating gate 5 is positioned with a thin tunnel oxide film 18 interposed, on the channel 8. An upper surface of floating gate 5 is covered by a control gate 6 with a thin ONO (oxide nitride oxide) film 20 interposed, thus providing a capacitance. Further, at an upper portion of control gate 6, a gate electrode 7 is formed for taking out electrical charges from the control gate.
FIG. 10 is a cross sectional view taken along the line Xxe2x80x94X of FIG. 6. Referring to the figure, in the cross section along the bit line direction of the non-volatile semiconductor memory device, source regions 2a and drain regions 4 are arranged alternately at the main surface of semiconductor substrate 1, and between each of these regions, a channel 8 is positioned. Immediately above the channel 8, a floating gate 5 is positioned with a thin tunnel oxide film 18 interposed, and on an upper surface of floating gate 5, control gate 6 and gate electrode 7 are positioned, with the aforementioned ONO film 20 interposed. The non-volatile semiconductor memory device having such a structure is thus formed.
The method of forming the source diffusion layer region in the non-volatile semiconductor memory device having the above described structure is as follows. FIGS. 11A and 11B are cross sections representing the method of forming the non-volatile semiconductor memory device having the above described structure. First, a trench is formed at the main surface of semiconductor substrate 1, the trench is filled with an isolation film to form a trench isolation region 17, and the isolation film of that portion of the trench isolation region 17 which overlaps the source diffusion layer region 2 is removed by self align source etching. Consequently, the main surface of semiconductor substrate 1 at a portion that will be source diffusion layer region 2 comes to have such a cross sectional structure as shown in FIG. 11A, which includes recesses and protrusions.
Thereafter, an n type impurity such as arsenic is ion-implanted to the portion that will be source diffusion layer region 2, from a direction approximately at a right angle with the main surface of semiconductor substrate 1 (from the direction of the arrow D in the figure). As the upper surface of the protrusions are positioned approximately at a right angle with respect to the direction of ion implantation, a deep source region 2a is formed immediately therebelow. Further, as the bottom surface of the recesses is positioned approximately at a right angle with respect to the direction of ion implantation, a deep source diffusion layer interconnection 2b1 is formed immediately therebelow. A sidewall of the recessed portion formed by removing trench isolation described above has an inclination angle of approximately 90xc2x0. Therefore, implanted ions do not much diffuse, and hence only a shallow source diffusion layer interconnection 2b2 is formed at the surface. As these source regions are formed continues to each other, a so-called source line is formed (FIG. 11B). When the sidewall of the recessed portion is particularly steep, the source diffusion layer interconnection 2b2 is not formed at the sidewall portion, resulting in a discontinuous source line.
In the non-volatile semiconductor memory device described above, the main surface of the semiconductor substrate has recesses and protrusions at the cross section of the source diffusion layer region. Therefore, it is difficult to introduce ions to the sidewalls of the trenches by conventional vertical ion implantation to the main surface of the semiconductor substrate, so that the source diffusion layer region comes to have an unstable and discontinuous structure, of which resistance becomes high. Because of this high resistance, voltage drops as a current flows through the source diffusion layer region, and therefore, at a cell positioned far from a portion where the potential of the source region is fixed, efficiency of writing decreases. Further, a threshold voltage Vth becomes lower in order to obtain read current at the time of erasure, resulting in excessive erasure.
Japanese Patent Laying-Open No.2000-36546 discloses a structure in which a trench sidewall is formed with an angle larger than the angle of ion implantation to provide a source diffusion layer region of low resistance. This method, however, is disadvantageous to miniaturization of the semiconductor device, as the trench sidewall is inclined. Though it is described in this reference that the angle of ion implantation is set to 7xc2x0, such setting is specified simply to improve implantation efficiency by preventing piercing of the ions, and not to lower the resistance of the source diffusion layer region.
Japanese Patent Laying-Open No. 2000-91545 discloses a technique in which a trench is formed as two-stepwise trench so as to lower the resistance of the source diffusion layer region. Such a structure, however, significantly increases the number of process steps, and hence increases manufacturing cost.
An object of the present invention is to provide a non-volatile semiconductor memory device having such a structure that does not limit miniaturization even when trench isolation suitable for miniaturization is used and that can lower resistance of a source diffusion layer region with the number of process steps not much increased, and to provide a manufacturing method thereof.
The non-volatile semiconductor memory device in accordance with the present invention has a source diffusion layer region formed continuously at a main surface of the semiconductor substrate, wherein the main surface of the semiconductor substrate at the source diffusion layer region has recesses and protrusions repeated continuously and alternately in a cross section parallel to a direction of extension of the source diffusion layer region; the source diffusion layer region includes, when the semiconductor substrate is viewed two-dimensionally, a first source diffusion layer region formed from an upper surface of each protrusion toward the depth direction of the semiconductor substrate and a second source diffusion layer region formed, when the semiconductor substrate is viewed to-dimensionally, from a bottom surface of each recess toward the depth direction of the semiconductor substrate; and depth from the upper surface of the protrusion to the bottom surface of the first source diffusion layer region is the same as or larger than the depth from the upper surface of the protrusion to the bottom surface of the recess.
Because of this structure, the source diffusion layer region surely becomes continuous and comes to have a large cross sectional area in the cross section vertical to the direction of extension thereof. Thus, the source diffusion layer region comes to have lower resistance. Further, as the source diffusion layer region extends down to the bottom surface of the protrusion, source regions are coupled through shortest path with each other, which leads to further decrease of the resistance of the source diffusion layer region.
Desirably, in the non-volatile semiconductor memory device of the present invention described above, in a first source diffusion layer region, for example, impurity concentration distribution of a linear portion coupling lower ends of sidewalls of each protrusion has one peak between a mid point of the linear portion and one of the lower ends, and another peak between the mid point and the other lower end.
By controlling conditions of ion implantation to realize the above described feature, a source diffusion layer region can be formed which is surely continuous at the end portions and at the middle portion of the protrusion.
In the non-volatile semiconductor memory device described above, in the first source diffusion layer region for example, the impurity concentration distribution of the linear portion connecting the lower ends of sidewalls forming each protrusion may have a peak near the mid point thereof.
By controlling conditions of ion implantation to realize the above described feature, a source diffusion layer region which is surely continuous at the end portions and at the middle portion of the protrusion is formed.
In the non-volatile semiconductor memory device of the present invention described above, the protrusions and recesses at the main surface of the semiconductor substrate may be formed by trench isolation.
The present invention is particularly effective when the recesses and protrusions at the main surface of the semiconductor substrate are steep recesses and protrusions formed by using trench isolation. Even when LOCOS process is used for isolation, application of the present invention is effective if end portions of the isolation film is formed steep.
A method of manufacturing a non-volatile semiconductor memory device in accordance with the present invention is for manufacturing a nonvolatile semiconductor memory device having a source diffusion layer region formed continuously at the main surface of a semiconductor substrate, which includes: a first step of forming a plurality of element isolating regions parallel to each other by forming isolation films at the main surface of the semiconductor substrate; a second step of removing the isolation films at portions that will be source diffusion layer regions among the element isolating regions to provide recesses and protrusions at the main surface of the semiconductor substrate; and a third step of oblique ion implantation under such a condition that in a cross section parallel to a direction of extension of the source diffusion layer region, a mid point of a line connecting lower ends of sidewalls of each protrusion forms a part of the source diffusion layer region.
By the above described manufacturing method, the structure described above can be obtained. More specifically, ions introduced to an arbitrary one point at the main surface of the semiconductor substrate in the third step radially expands from this point as a center, and forms the source diffusion layer region. By performing oblique ion implantation under such a condition that ions introduced to the lower end of the sidewall of each protrusion reach the middle point of the line connecting the lower ends of the sidewall of the protrusion, the depth of the bottom surface of the first source diffusion layer region mentioned above surely becomes deeper than the bottom surface of the recessed portion. Accordingly, the resistance of the source diffusion layer region can be significantly lowered as compared with the prior art.
Desirably, the third step of the method of manufacturing a nonvolatile semiconductor memory device in accordance with the present invention includes: the step of ion implantation obliquely from above to the main surface of the semiconductor substrate at an angle smaller than an acute one of the angles formed by a line connecting a lower end of one sidewall of each recess and an upper end of the opposite sidewall of the recess with a line vertical to the main surface of the semiconductor substrate, to form a part of the source diffusion layer region; and a step of ion implantation obliquely to the main surface of the semiconductor substrate from a direction in line-symmetry with the vertical line being the axis, at the same angle as the former ion implantation, to form remaining part of the source diffusion layer region.
By performing oblique ion implantation twice to the sidewalls of the protrusion as in the present method of manufacturing, the source diffusion layer region can surely be formed in the protrusion. The twice oblique ion implantation operations of the fixed type should desirably be performed from directions in line-symmetry with the vertical line to the main surface of the semiconductor substrate being the axis of symmetry.
The method of manufacturing a non-volatile semiconductor memory device of the present invention may additionally include a step of introducing ions approximately vertically to the main surface of the semiconductor substrate.
This additional step ensures formation of the source diffusion layer region. This method of manufacturing is effective when deeper source diffusion layer regions are to be formed at the upper surface of the protrusions and the bottom surface of the recesses.
In the method of manufacturing a non-volatile semiconductor memory device of the present invention, the method of forming an isolation film in the first step, for example, may be trench isolation.
When the method of forming the insulating film is trench isolation as in the manufacturing method described above, steep recesses and protrusions are formed, and therefore, application of the present invention is particularly effective. Even when the method of forming an isolation film utilizing LOCOS process is used, application of the present invention is effective particularly when the recesses and protrusions are steep.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.